The present invention relates to improved composite materials and the melt infiltration methods for producing the same. Specifically, the present invention relates to silicon carbide composites wherein preferably at least a portion of the silicon carbide is produced by reactive infiltration.
Silicon carbide composites have been produced by reactive infiltration techniques for more than thirty-five years. In general, such a reactive infiltration process comprises contacting molten silicon with a porous mass containing silicon carbide plus carbon in a vacuum or an inert atmosphere environment. A wetting condition is created, with the result that the molten silicon is pulled by capillary action into the mass, where it reacts with the carbon to form additional silicon carbide. This in-situ silicon carbide typically is interconnected. A dense body usually is desired, so the process typically occurs in the presence of excess silicon. The resulting composite body thus comprises silicon carbide and unreacted silicon (which also is interconnected), and may be referred to in shorthand notation as Si/SiC. The process used to produce such composite bodies is interchangeably referred to as xe2x80x9creaction formingxe2x80x9d, xe2x80x9creaction bondingxe2x80x9d or xe2x80x9creactive infiltrationxe2x80x9d.
In one of the earliest demonstrations of this technology,Popper (U.S. Pat. No. 3,275,722) produced a self-bonded silicon carbide body by infiltrating silicon into a porous mass of silicon carbide particulates and powdered graphite in vacuo at a temperature in the range of 1800 to 2300 C.
Taylor (U.S. Pat. No. 3,205,043) also produced dense silicon carbide bodies by reactively infiltrating silicon into a porous body containing silicon carbide and free carbon. Unlike Popper, Taylor first made a preform consisting essentially of granular silicon carbide, and then he introduced a controlled amount of carbon into the shaped mass. In one embodiment of his invention, Taylor added the carbon in the form of a carbonizable resin, and then heated the mass containing the silicon carbide and infiltrated resin to decompose (carbonize) the resin. The shaped mass was then heated to a temperature of at least 2000 C in the presence of silicon to cause the silicon to enter the pores of the shaped mass and react with the introduced carbon to form silicon carbide.
Hillig and his colleagues at the General Electric Company took a different approach, where fibrous versions of Si/SiC composites were produced by reactively infiltrating carbon fiber preforms.
More recently, Chiang et al. (U.S. Pat. No. 5,509,555) discloses the production of silicon carbide composite bodies through the use of a silicon alloy infiltrant. The preform to be infiltrated by the alloy can consist of carbon or can consist essentially of carbon combined with at least one other material such as a metal like Mo, W, or Nb; a carbide like SiC, TiC, or ZrC; a nitride like Si3N4, TiN or AlN; an oxide like ZrO2 or Al2O3; or an intermetallic compound like MoSi2 or WSi2, or mixtures thereof. The liquid infiltrant includes silicon and a metal such as aluminum, copper, zinc, nickel, cobalt, iron, manganese, chromium, titanium, silver, gold, platinum and mixtures thereof.
In a preferred embodiment of the Chiang et al. invention, the preform can be a porous carbon preform, the liquid infiltrant alloy can be a silicon-aluminum alloy containing in the range of from about 90 at % to about 40 at % silicon and in the range of from about 10 at % to about 60 at % aluminum and the carbon preform can be contacted with the silicon-aluminum alloy at a temperature in the range:of from about 900 C to about 1800 C for a time sufficient so that at least some of the porous carbon is reacted to form silicon carbide. Upon cooling, the dense composite formed thereby can be characterized by a phase assemblage comprising silicon carbide and at least one phase such as silicon-aluminum alloy, a mixture of silicon and aluminum, substantially pure aluminum or mixtures thereof.
One problem with infiltrating multi-constituent liquids into preforms containing large fractions of carbon is that the infiltrant chemistry can change dramatically over the course of infiltration, as well as from one location to another within the preform. Table 3 of Chiang et al. demonstrates this point. There, the infiltrant started out as being about 54 at % Si, 46 at % Cu, but after infiltration into a carbon preform, it was substantially 100% Cu. Such drastic compositional changes can make processing difficult; this same Table revealed that when the infiltrant alloy started out at about 30 at % Si, 70 at % Cu, pressure was required to achieve infiltration. Pressure infiltrations require much more complex and expensive equipment than do pressureless infiltration techniques, and usually are more limited in the size and shape of the parts that can be produced thereby. Thus, while the present invention is not limited to pressureless systems, unless otherwise noted, the infiltrations of the present invention refer to those not requiring the application of pressure.
Chiang et al. state that their method allows production of composites very near net-shape without a need for additional machining steps. They describe a number of non-machining techniques for removing the residual, unreacted liquid infiltrant alloy remaining on the reacted preform surface. Specifically, Chiang et al. state that following infiltration, the composite body may be heated to a temperature sufficient to vaporize or volatilize the excess liquid alloy on the surface. Alternatively, the reacted preform may be immersed in an etchant in which the excess unreacted liquid infiltrant is dissolved while the reacted preform is left intact. Still further, the reacted preform may be contacted with a powder that is chemically reactive with the unreacted liquid infiltrant alloy such as carbon, or a metal like Ti, Zr, Mo or W.
In U.S. Pat. No. 5,205,970, Milivoj Brun et al. also are concerned with removing excess infiltrant following production of silicon carbide bodies by an infiltration process. Specifically, Brun et al. contact the reaction formed body with an infiltrant xe2x80x9cwicking meansxe2x80x9d such as carbon felt. More generally, the wicking means may comprise porous bodies of infiltrant wettable materials that are solid at the temperature at which the infiltrant is molten. Preferably, the wicking means has capillaries that are at least as large or larger than the capillaries remaining in the reaction formed body. Thus, infiltrant in the reaction formed body that is filling porosity remains in the reaction formed body instead of being drawn into the wicking means and leaving porosity in the reaction formed body.
The xe2x80x9cwicking meansxe2x80x9d solution of Brun et al. to the problem of removing excess adhered silicon, while perhaps effective, nevertheless requires the additional processing steps of contacting the formed composite body with the wicking means and re-heating to above the liquidus temperature. What is needed is a means for eliminating or at least minimizing the degree of residual infiltrant adhered to the formed silicon carbide composite body.
It is an object of the present invention to produce a silicon carbide composite body to near-net shape, thereby minimizing the amount of grinding and/or machining necessary to achieve the required dimensions of the finished article.
It is an object of the present invention to produce a silicon carbide composite body of improved toughness.
It is an object of the present invention to produce a silicon carbide composite body of increased thermal conductivity.
It is an object of the present invention to produce a silicon carbide composite body whose thermal expansion coefficient is above that of silicon carbide, and tailorable.
It is an object of the present invention to produce a silicon carbide composite body at temperatures that are above, but only modestly above, the liquidus temperature of the silicon-bearing infiltrant material.
It is an object of the present invention to produce a silicon carbide composite body at a temperature that is substantially less than the melting point of pure silicon.
It is an object of the present invention to produce a silicon carbide composite body without having to resort to boron-containing barrier materials or expensive molds to control the extent of infiltration.
These objects and other desirable attributes of the present invention are accomplished through careful control of a number of the processing conditions employed in making composite bodies by reactive infiltration. In terms of the present invention, the most important of these processing conditions is infiltrant chemistry. Specifically in accordance with the present invention, the infiltrant material comprises at least two constituents, and at least one of the constituents comprises silicon.
It has been noted that silicon undergoes a net volume expansion of about 9 percent upon solidification. Thus, in accordance with one aspect of the present invention, by alloying the silicon with a material that undergoes a net volume shrinkage upon solidification, it is possible to produce a silicon carbide composite body having a residual infiltrant component that undergoes substantially no net volume change upon solidification. Thus, production of silicon carbide composite bodies that exhibit neither solidification porosity nor solidification exuding of the infiltrant component can be realized.
Carbon is frequently added to the permeable mass to enhance infiltration. (Unless otherwise noted, from hereon the term xe2x80x9cpermeable massxe2x80x9d will be understood to include the term xe2x80x9cpreformxe2x80x9d.) One ramification of such alloying, however, is the change that takes place in the chemical composition of the infiltrant as it infiltrates the permeable mass or preform and as the silicon constituent of the infiltrant alloy reacts with the carbon contained therein to produce silicon carbide. Accordingly, the present inventors have discovered the significance and importance of keeping the carbon content of the permeable mass to be infiltrated at relatively low levels. Preferably, the amount of free carbon in the permeable mass is kept as low as necessary to accomplish complete infiltration in a reliable manner but without unduly compromising the binder qualities of the carbon when preforms (e.g., self-supporting permeable masses) are used. This way, large bodies can be infiltrated with minimal changes in the infiltrant alloy composition, thereby resulting in a silicon carbide composite body having a dispersed alloy phase of relatively uniform composition throughout the body.
The use of a multi-constituent infiltrant composition has additional advantages beyond the ability to produce composite bodies whose alloy component has zero or near-zero volumetric change (swelling or contraction) upon solidification.
For instance, and in another major aspect of the present invention, the alloying of silicon infiltrant with one or more different elemental constituents can substantially depress the melting point of the infiltrant. Desirable alloying elements in this regard include aluminum, beryllium, copper, cobalt, iron, manganese, nickel, tin, zinc, silver and gold. The lowered melting or liquidus temperatures permit the infiltration to be conducted at lower temperatures, e.g., in a range of about 800 C to about 1400 C. For example, when the infiltrant comprises a silicon-aluminum alloy, it is possible to infiltrate a porous mass comprising some elemental carbon at a temperature in the range of about 1100 to about 1300 C. By way of comparison, when the infiltrant consists essentially of silicon, the temperature must be maintained at least above the silicon melting point of about 1412 C, and often substantially above the melting point so that the melt is sufficiently fluid. One of the most important consequences of being able to operate at lower temperatures is the discovery that at the lower temperatures, the infiltration is more reliably terminated at the boundaries of the permeable mass. Further, instead of having to use expensive graphite molds to support the permeable mass and to confine the liquid infiltrant, cheaper materials such as a loose mass of ceramic particulate may be used. Thus, the ability to conduct infiltrations at lower temperatures gives operators more control over the process, not to mention saving time and energy.